The present invention relates generally to implantable medical devices and methods for cardiac stimulation. More particularly, the present invention pertains to implantable medical devices and methods that employ mode switching in cardiac stimulation.
Generally, in the human heart, the sinus (or sinoatrial (SA) node typically located near the junction of the superior vena cava and the right atrium) constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrio-ventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract with the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
The above action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, and then relax and fill. One-way valves, between the atrial and ventricular chambers on the right and left sides of the heart, and at the exits of the right and left ventricles, prevent backflow of the blood as it moves through the heart and the circulatory system. This sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed sinus rhythm. This capacity to produce spontaneous cardiac impulse is called rhythmicity. Some other cardiac tissues possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
Disruption of the natural pacemaking and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. A pacemaker is a medical device which delivers electrical pulses to an electrode that is implanted adjacent to or in the patient""s heart to stimulate the heart so that it will contract and beat at a desired rate. If the body""s natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body""s oxygen-demanding tissues. However, when the body""s natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart.
Implantable pacemakers are typically designed to operate using various different response methodologies, such as, for example, nonsynchronous or asynchronous (fixed rate), inhibited (stimulus generated in the absence of a specified cardiac activity), or triggered (stimulus delivered in response to a specific hemodynamic parameter). Generally, inhibited and triggered pacemakers may be grouped as xe2x80x9cdemandxe2x80x9d-type pacemakers, in which a pacing pulse is only generated when demanded by the heart. To determine when pacing is required by the pacemaker, demand pacemakers may sense various conditions such as heart rate, physical exertion, temperature, and the like. Moreover, pacemaker implementations range from the simple fixed rate, single chamber device that provides pacing with no sensing function, to highly complex models that provide fully-automatic dual chamber pacing and sensing functions. For example, such multiple chamber pacemakers are described in U.S. Pat. No. 4,928,688 to Mower entitled xe2x80x9cMethod and Apparatus for Treating Hemodynamic Dysfunction,xe2x80x9d issued May 29, 1990; U.S. Pat. No. 5,792,203 to Schroeppel entitled xe2x80x9cUniversal Programmable Cardiac Stimulation Device,xe2x80x9d issued Aug. 11 , 1998; U.S. Pat. No. 5,893,882 to Peterson et al. entitled xe2x80x9cMethod and Apparatus for Diagnosis and Treatment of Arrhythmias,xe2x80x9d issued Apr. 13, 1999; and U.S. Pat. No. 6,081,748 to Struble et al. entitled xe2x80x9cMultiple Channel, Sequential Cardiac Pacing Systems,xe2x80x9d issued Jun. 27, 2000.
Because of the large number of options available for pacer operation, an industry convention has been established whereby specific pacer configurations are identified according to a code comprising multiple letters (generally, three to four letters, although a fifth coded position may also be used). The most common configuration codes comprise either three or four letters, as shown in Table I below. For simplicity, the fifth coded position is omitted. Each code can be interpreted as follows:
For example, a DDD pacer paces either chamber (atrium or ventricle) and senses in either chamber. Thus, a pacer in DDD mode, may pace the ventricle in response to electrical activity sensed in the atrium. A VVI pacer paces and senses in the ventricle, but its pacing is inhibited by spontaneous electrical activity of the ventricle, also referred to as intrinsic ventricular activity (i.e., the ventricle paces itself naturally). In VVIR mode, ventricle pacing is similarly inhibited upon determining that the ventricle is naturally contracting. With the VVIR mode, the pacer""s pacing rate, however, in the absence of naturally occurring pacing, is modulated by the physical activity level of the patient. Pacers commonly include accelerometers to provide an indication of the patient""s level of physical activity.
As illustrated in the table above, it may be desirable to sense in one cardiac chamber (e.g., detect electrical activity represented of contraction of the chamber and referred to as a xe2x80x9csensed eventxe2x80x9d) and, in response, pace (referred to as a xe2x80x9cpaced eventxe2x80x9d) in the same or different chamber. It also may be desirable to pace at two electrode locations following a sensed event. For example, patients with abnormally fast atrial rhythms (referred to as atrial tachyarrhythmias) are often treated with pacemakers that include an electrode in each of the two atrial chambers and a third electrode in the right ventricle. Both atrial chambers usually are paced following a sensed event in either chamber. Various pacemaker protocols may be used.
Further, for example, some patients, like heart failure patients, are often treated with bi-ventricular pacemakers that include an electrode in each of the two ventricular chambers, and also possible a third electrode in the right atrium. Both ventricular chambers usually are paced following a sensed or paced atrial event.
In the context of dual chamber pacing, a variety of mode switching features have been developed which detect an excessively rapid atrial rhythm and in response cause the pacemaker to switch from an atrial synchronized pacing mode such as DDD to a nonsynchronized mode such as VVI or DDI. Such mode switching features are disclosed in U.S. Pat. No. 5,144,949 to Olson entitled xe2x80x9cDual Chamber Rate Responsive Pacemaker With Automatic Mode Switching,xe2x80x9d issued Sep. 8, 1992; U.S. Pat. No. 5,318,594 to Limousin et al. entitled xe2x80x9cDDD Type Cardiac Pacemaker Having Automatic Operating Mode Switching,xe2x80x9d issued Jun. 7, 1994; U.S. Pat. No. 4,944,298 to Sholder entitled xe2x80x9cAtrial Rate Based Programmable Pacemaker With Automatic Mode Switching Means,xe2x80x9d issued Jul. 31, 1990; U.S. Pat. No. 4,932,406 to Berkovits entitled xe2x80x9cDual Chamber Rate Responsive Pacemaker,xe2x80x9d issued Jun. 12, 1990; and U.S. Pat. No. 5,292,340 to Crosby et al. entitled xe2x80x9cPhysiologically-Calibrated Rate Adaptive, Dual Chamber Pacemaker,xe2x80x9d issued Mar. 8, 1994. In such devices, the primary purpose of the mode switch is to prevent the pacemaker from tracking a non-physiologic atrial rate.
Generally, mode switching is generally in most dual chamber pacemakers. Such mode switching typically changes the mode of pacing therapy during periods of accelerated atrial arrhythmias such as, for example, SVT (supra ventricular tachycardia), PAF (paroxysmal atrial flutter), and AF (atrial fibrillation). For example, mode switching may change the dual chamber pacing mode from DDD to DDI, DDDR to DDIR, VDD to VVI, or VDDR to VVIR.
During such episodes of mode switching due to periods of accelerated atrial arrhythmias such as SVT/PAF/AF, the pacemaker will revert to a lower rate (LR) of pacing (or a sensor-driven pacing rate or frequency in rate modulating operating modes such as DDIR or VVIR). In many cases, the LR is programmed below that of the intrinsic rate of the patient""s sinus rhythm. For example, the LR may be 60 ppm when the sinus rhythm of the patient is 70 bpm. As such, with regard to patients with ventricular dysfunction (e.g., heart failure), because such patients are inactive due to their severe conditions, the heart rate may be paced at an insufficient low pacing rate, i.e., LR.
Therefore, such mode switching may result in insufficient pacing rate and cardiac output. For example, during mode switching periods, as described above, the pacemaker may pace the heart at the LR in a mode such as DDI(R) or VVI(R). DDIR behaves much like VVIR in the case of atrial tachyarrhythmias. This pacing LR is typically too slow to guarantee sufficient cardiac output in heart failure patients.
In addition to the potential lower cardiac output due to pacing at the LR, reduced cardiac output may also occur due to the atrial arrhythmia and loss of atrial contribution to ventricular filling. For example, all atrial contribution (e.g., xe2x80x9catrial kickxe2x80x9d) to ventricular filling may be lost during atrial arrhythmia. As such, stroke volume becomes reduced, e.g., reduced by 20-25%, because cardiac output=(heart rate)(stroke volume). Due to the above, such reduced cardiac output may be inadequate for the patient.
Further, during periods of accelerated atrial arrhythmias (e.g., SVT/PAF/AF), AV conduction often occurs irregularly. Such irregular AV conduction may result in irregular intrinsic ventricular response (e.g., ventricular response rates of 100 bpm due to the attempt of the ventricular chamber to respond intrinsically to the accelerated arrhythmias to the LR of 60 ppm when no intrinsic ventricular response is detected and the ventricular chamber is paced at LR).
Yet further, in bi-ventricular pacing for heart failure patients, continuous pacing therapy should be maintained. During mode switching, there may be a loss of such continuous bi-ventricular pacing therapy.
Table II below lists U.S. Patents relating to multiple chamber pacing apparatus and mode switching techniques and methods.
All references listed in Table II, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table II and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table II, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to implantable medical device pacing techniques and, in particular, mode switching used in conjunction with such pacing techniques. One of such problems involves the provision of insufficient pacing rate and cardiac output during mode switching periods. Further, for example, other problems involve the occurrence of irregular AV conduction during accelerated atrial arrhythmias and mode switching periods that often result in irregular intrinsic ventricular response. In addition, for example, in bi-ventricular pacing for heart failure patients, during mode switching periods upon detection of accelerated atrial arrhythmias, pacing therapy may not be continuous.
In comparison to known mode switching techniques, various embodiments of the present invention may provide one or more of the following advantages. For example, the highest level of continued ventricular therapy, e.g., bi-ventricular pacing therapy, during mode switching periods due to accelerated atrial arrhythmias, is ensured. Further, an elevated pacing rate counteracts the absence of atrial contribution to ventricular filling in patients during periods of atrial arrhythmias. Yet further, the present invention provides for interaction in the mode switching period with a rate response activity sensor indicated rate to provide for more appropriate pacing rates when a patient is undertaking greater activity, e.g., exercise. In general, by making adjustments to lower rate pacing during mode switching, such that an elevated compensatory rate is provided, the present invention provides the advantage of providing sufficient cardiac output during episodes of accelerated atrial arrhythmias.
Some embodiments of the present invention include one or more of the following features: detection of a period of accelerated atrial arrhythmias; switching from a first pacing mode to a second pacing mode upon detection of a period of accelerated atrial arrhythmias; provision of a first pacing mode (e.g., DDD, DDDR, VDD, or VDDR pacing mode) that paces at least one ventricle based on sensed atrial activity and a second pacing mode (e.g., DDI, DDIR, VVI, or VVIR pacing mode) that paces the at least one ventricle based on sensed ventricular activity at a predetermined lower rate with such pacing inhibited based on intrinsic ventricular activity; adjusting a lower rate to an elevated adjusted lower rate upon switching from a first pacing mode to a second pacing mode such that pacing of the at least one ventricle is not inhibited based on intrinsic ventricular activity; adjusting a lower rate to an elevated adjusted lower rate based on R-R intervals measured during a ventricular response detection time window associated with mode switching; adjusting a lower rate based on at least the fastest R-R interval measured during a ventricular response detection time window; limiting the elevated adjusted lower rate based on a programmed maximum pacing rate; taking into consideration an activity sensor indicated pacing rate when determining an appropriate rate; decelerating from an elevated adjusted lower rate towards a predetermined basic pacing rate that is as fast or faster than the predetermined or programmed lower rate; monitoring ventricular activity during the deceleration period and readjusting the elevated adjusted lower rate upon detection of an intrinsic ventricular event and further decelerating the readjusted elevated lower rate during a reinitiated deceleration period; and continuing deceleration to a predetermined pacing rate if no intrinsic ventricular events are detected and thereafter continuing to use a predetermined basic pacing rate until either an intrinsic ventricular event is detected and a readjusted elevated lower rate is reset for deceleration or mode of operation is switched back.
Still further, some embodiments of the present invention include one or more of the following features: pacing generator circuitry operable to generate pacing pulses at one or more pacing rates during at least first and second pacing modes; a first pacing mode that paces at least one ventricle based on sensed atrial activity (e.g., DDD, DDR, VDD, or VDDR); a second pacing mode that paces the at least one ventricle based on sensed ventricular activity at a predetermined lower rate (e.g., a programmed rate) with such pacing inhibited based on intrinsic ventricular activity (e.g., DDI, DDIR, VVI, or VVIR pacing mode); sensing circuitry operable to sense atrial and ventricular activity; a pacing controller operable to switch from a first pacing mode to a second pacing mode upon detection of a period of accelerated atrial arrhythmia based on information from sensing circuitry; a pacing controller operable to at least initially upon switching from a first pacing mode to a second pacing mode adjust a predetermined lower rate to an elevated adjusted lower rate such that pacing of at least one ventricle is not inhibited based on detected intrinsic ventricular activity; a pacing controller that is operable to adjust a predetermined lower rate to an elevated adjusted rate based on R-R intervals measured during a ventricular response detection time window associated with mode switching; a pacing controller that is operable to limit an elevated adjusted lower rate based on a programmed maximum pacing rate; a pacing controller that is operable to control the pacing rate based on an activity sensor indicated pacing rate; a pacing controller that is operable to decelerate an elevated adjusted lower rate towards a predetermined basic pacing rate (e.g., a programmed rate) that is as fast or faster than the predetermined lower rate; a pacing controller that is operable to readjust an elevated adjusted lower rate during a deceleration window based on intrinsic ventricular activity and to control deceleration of the readjusted elevated lower rate during a reinitiated deceleration period; and a pacing controller that is operable to continue deceleration to a predetermined basic pacing rate if no intrinsic ventricular activity is sensed during a deceleration window and thereafter continue to use the predetermined basic pacing rate until either intrinsic ventricular activity is sensed and a new readjusted elevated lower rate is reset for deceleration during another reinitiated deceleration period or the mode of operation is switched back.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.